For connecting pipes, such as plastic pipes, plug-type sleeves are often used, into which sleeves the end sections of the pipes to be connected are fitted from two opposite sides and then are snugly secured therein. Alternatively, the plug-type sleeves can also be designed on the end of a pipe.
In the case of a widely available type of such plug-type sleeves, the securing is done by means of O-rings made of elastomeric material, which at the same time also provide for the necessary sealing of the connection. The O-rings are located in circumferential grooves of the plug-type sleeves and are squeezed radially during the fitting of the end sections of the pipe or pipes to be connected between an inner wall of the plug-type sleeve and the outer surfaces of the end sections, so that they rest snugly on the outer surfaces of the end sections and hold tightly by friction the end sections of the pipes.
Such plug-type sleeves are very simple structurally and no additional aids are necessary for connecting the pipes. However, such plug-type sleeves are often not used because they require a relatively high amount of energy in order to fit the end sections of the pipes that are to be connected into the plug-type sleeve, since the necessarily strong squeezing of the O-rings offers considerable resistance to the fitting of the end sections. By suitable sizing, the necessary squeezing of the O-rings and thus the resulting resistance can be reduced, but then at the same time, the achievable extraction strength is also decreased, which in many applications is undesirable or unacceptable.
In connection with electric cables, plug-type sleeves (e.g., for cable fittings), are used for traction relief. In most cable fittings, which must guarantee sealing and a specific extraction force, an O-ring that consists of soft rubber is arranged in the plug-type sleeve, which is axially clamped by means of a union nut and as a result can be squeezed radially inward, whereby it presses on the cable jacket and thus holds the cable tightly. For pipe connections, such plug-type sleeves with union nuts are only conditionally suitable, however.